Hi and welcome to the club!
No tools are required to maintain as such but to strip parts, in the event you need to, you will likely need some screwdrivers, barrel nut tool, and a magazine valve tool.
The width of the buffer tube is mil spec but the style of it is commercial spec - I have put a mil spec Magpul CTR stock on and it fits lovely! Most should fit, some might be wobbly or very tight. Trial and error.
Its not real steel spec - has to be MWS specific, but PTS do a MWS Radian charging handle (have it on mine and its very nice)
Yes. I have a Magpul K2 on mine - have to use the original MWS bolt.
The thread on the receiver is AEG spec, so anything for an AEG should fit, but you will likely need to buy the barrel adapter for your barrel to work with an alternative setup.
It replaces the rubber nub in the hop chamber - it gives stability - worth doing for lifting any weight.